stereoisomerism

Stereoisomerism is one of two branches of isomerism, the other being structural isomerism. Structural isomers are the more common form of isomers and arise from the different possibilities in which a group of atoms can bond. For example, a functional group such as -OH can often bond to various points in a carbon chain. While the resulting alcohols will have the same chemical formula, they differ in structure.
Stereoisomers are a more subtle form of isomer. They occur in two different forms: geometric isomers and enantiomers.
Geometric isomers arise where double bonds are present. Due to a higher electron density in double bonds as compared to single bonds, the atoms joined by a double bond are not free to rotate. Because of this restriction, two forms of the molecule are possible, and these are differentiated with the prefixes cis and trans.

Cl Cl
\ /
C===C
/ \
H H

This is cis-1,2-dichloroethene. The molecule which has two similar groups on the same side of the double bond is known as the cis-isomer.

Cl H
\ /
C===C
/ \
H Cl

This is trans-1,2-dichloroethene. The similar groups are on opposite sides of the double bond, so this is known as the trans-isomer.

The chemical properties are usually similar between stereoisomers. However, physical properties can be affected. In the above example, the cis-isomer is less symmetrical than the trans-isomer and thus the cis-isomer possesses a dipole. As a result, the boiling point of cis-1,2-dichloroethene is 60 degrees while trans-1,2-dichloroethene boils at 48 degrees.